1. Field of the Invention
The present invention is related to handovers in a communication system and particularly, but not exclusively, to mobile telephone station handovers in an IP-based telecommunications network.
2. Description of the Related Art
Prior art office-based communications systems usually operate conventional fixed-line telephone units linked via an internal switchboard or PBX (private branch exchange). Such fixed-line systems are able to provide relatively high voice quality. However, user mobility is severely impaired.
The advent of digital mobile technologies such as GSM (Global system for mobile communications) however, means that mobile systems can now provide at least the equivalent voice quality as fixed-line systems. Mobile systems also allow greater freedom of movement for the user within the office than do fixed-line systems.
WIO (Wireless Intranet Office) is a proprietary communications system developed by the applicants which introduces the concept of utilising mobile telephone units, such as conventional GSM mobile stations, in an office environment. The system makes use of a known concept called Internet Telephony or Voice-over-IP. (IP is internet protocol).
Voice-over-IP is a technology which allows audio, data and video information to be transmitted over existing IP-based Local or Wide Area Networks or the Internet. The technology thus provides for convergence and integration of three different media types over the same network.
Prior to the advent of Voice-over-IP, offices often operated three separate networks for the transmission of these media types. As indicated above, fixed-line telephone systems coupled to an in-house PBX provided for voice communication, an officebased LAN (local area network) or Intranet (i.e. a packet-switched internal network), comprising computer terminals linked via network cards and under the control of a server station, provided for the transmission of xe2x80x9cconventionalxe2x80x9d computer data and video cameras linked to monitors via fixed line or remote transmission link provided for video communication.
Voice-over-IP effectively combines these three media types such that they can be transmitted simultaneously on the same packet-switched network or IP-router throughout the office environment and beyond the confines of the office.
In order to provide for such media convergence, Voice-over-IP often uses a specific ITU (International Telecommunication Union) standard protocol to control the media flow over the Intranet. One common standard protocol used in Voice-over-IP systems, and the one used in the WIO system, is termed H.323.
H.323 is an ITU standard for multimedia communications (voice, video and data) and allows multimedia streaming over conventional packet-switched networks. The protocol provides for call control, multimedia management and bandwidth management for both point-to-point (2 end-users) and multipoint (3 or more end-users) conferences. H.323 also supports standard video and audio codes (compression/decompression methods such as MPEG) and supports data sharing via the T.120 standard.
Furthermore, H.323 is network, platform and application independent allowing any H.323 compliant terminal to operate in conjunction with any other terminal.
The. H.323 standard defines the use of three further command and control protocols:
a) H.425 for call control;
b) Q.931 for call signalling; and
c) The RAS (Registrations, Admissions and Status) signalling function.
The H.425 control channel is responsible for control messages governing the operation of the H.323 terminal including capability exchanges, commands and indications. Q.931 is used to set up a connection between two terminals. RAS governs registration, admission and bandwidth functions between endpoints and gatekeepers which are discussed later.
For a H.323 based communication system, the standard defines four major components:
1. Terminal
2. Gateway
3. Gatekeeper
4. Multipoint Control Unit (MCU)
Terminals are the user end-points on the network, e.g. a telephone, mobile or fixed, or a fax unit or a computer terminal. All H.323 compliant terminals must support voice communications, but video and data support is optional.
Gateways connect H.323 networks to other networks or protocols. For an entirely internal communications network i.e. with no external call facility, gateways as such are not required. However, a circuit providing the internal function of a gateway would normally be present. References to a gateway in the present description refers to any circuit providing the necessary gateway functions for internal or external communication.
Gatekeepers are the control center of the Voice-over-IP network. It is under the control of a gatekeeper that most transactions (communication between two terminals) are established. Primary functions of the gatekeeper are address translation, bandwidth management and call control to limit the number of simultaneous H.323 connections and the total bandwidth used by those connections. An H.323 xe2x80x9czonexe2x80x9d is defined as the collection of all terminals, gateways and multipoint-control units (MCUxe2x80x94defined below) which are managed by a single gatekeeper.
Multipoint Control Units (MCU) support communications between three or more terminals. The MCU comprises a multipoint controller (MC) which performs H.425 negotiations between all terminals to determine common audio and video processing capabilities, and a multipoint processor (MP) which routes audio, video and data streams between terminals.
The conventional Voice-over-IP system described herein above normally utilise standard fixed-line telephone systems which are subject to the disadvantages outlined above, namely the lack of mobility and the lack of user commands.
The WIO concept takes Voice-over-IP further in that it provides for the use of conventional mobile telephone units, such as GSM mobile stations, within the Voice-over-IP system. To provide for such mobile communications within an intra-office communication network, WIO combines known Voice-over-IP, as described above, with conventional GSM-based mobile systems.
Thus, intra-office calls are routed through the office intranet and extra-office calls are routed conventionally through the GSM network. Such a system provides most or all of the features supported by the mobile station and the network such as telephone directories, short messaging, multiparty services, data calls, call barring, call forwarding etc. WIO, therefore, provides for integrated voice, video and data communications by interfacing an H.323-based voice-over-IP network with a GSM mobile network.
The WIO system is a cellular network, similar to the conventional GSM network and is divided into H.323 Zones as described above.
One H.323 Zone may comprise a number of cells. Two or more H.323 zones may be contained within an administrative domain. The allocation of H.323 zones to an administrative domain is an issue primarily concerning billing and is therefore not relevant to this invention.
Given the cellular nature of the WIO system, a major issue to be solved is that of handovers (sometimes known as handoff) i.e. the hand-off of control of a mobile station from a first cell in the network to a second cell in the network. A similar consideration applies to mobile stations in the conventional GSM network.
In such conventional GSM systems, the need for a handover of a mobile station to a different cell of the network is based on measurements made by the mobile station of the strength of signals transmitted from several base transceiver stations.
If the level of a signal transmitted by a base transceiver station, located in a different cell from that of the mobile, reaches a certain threshold level in relation to that of the base transceiver station located in the mobile station""s current cell, handover of the mobile station is required and a handover request is issued to the network controller (mobile switching center).
In a similar manner, a mobile station operating in the WIO system is able to determine its position within the WIO network by comparing the signal strengths of the signals received by several base stations, in different cells, in the network.
However, added complexities arise for handovers in the WIO system since a mobile unit operating therein is not only able to move between cells within the WIO system, but also between zones and even between the WIO system itself and an external GSM network.
It can be seen, therefore, that there are several different types of handovers which may need to be executed in the normal operation of a WIO system. These types of handovers are:
a) The handover of a mobile from one WIO cell to another whilst in communication with another mobile.
b) The handover of a mobile from one WIO zone to another whilst in communication with another mobile.
c) The handover of a mobile from a cell within the WIO system to a cell within an external GSM system while in communication with another mobile and vice versa.
Handovers according to the types listed above are also subject to the conditions under which any ongoing call is made, such as the instantaneous location of each mobile station and the location of each mobile station when the call was set up.
During the handover of a mobile station from a first cell to a second cell, it is quite possible that a certain amount of data will be lost since the communications links from the second cell may not be fully set up when the communications links from the first cell are disconnected. It is preferable, therefore, to provide a handover procedure wherein the communications links from the first cell are not disconnected until the communications links from the second cell are fully set up.
According to one aspect of the present invention there is provided a method of effecting handover in a cellular communications network of a first mobile station between a source gateway and a target gateway, the first mobile station being engaged in a call with a second mobile station using a destination gateway wherein the source gateway, the target gateway and the destination gateway are connected by a switched packet communication path for conveying call data packets, the method comprising: i) opening a target port at the destination gateway for communication with the target gateway; ii) routing call data packets from the first mobile station via the target gateway to the target port on the switched packet communication path; iii) detecting receipt of said call data packets at the target port; and then, responsive to such detection, iv) routing call data packets from the second mobile station via the destination gateway to the target gateway on the switched packet communication path.
Another aspect of the invention provides a cellular communications network comprising: a plurality of gateways connected via a switched packet communication path for conveying call data packets between the gateways, each gateway being associated with circuitry for converting RF data from a mobile station to call data packets for transmission via the switched packet communication path, and each gateway having a set of selectable ports for transmitting and receiving call data packets on the switched packet communication path, the network further comprising: a central controller connected to the switched packet communication path and operative responsive to a handover required indication from a mobile station operating in the network to selectively open and close said ports to implement handover of the mobile station from one of said gateways to another of said gateways.
For a better understanding of the present invention, and to show how the same may be carried into effect, the present invention will now be described in more detail with reference to the accompanying drawings in which: